This invention generally relates to ultraviolet light reflective coatings and particularly concerns such coatings containing barium sulphate and allowing the intrinsically high reflectance of this material in the near vacuum ultraviolet to be optimally employed.
Barium suphate is known to have intrinsically high reflectance over the entire spectrum of ultraviolet light, i.e., within the range of wavelengths between 2,000 and 4,000 Angstrom units (A.U.). For this reason, barium sulphate is frequently employed as a pigment in ultraviolet light reflective coatings. Heretofore, such coatings have typically employed as a binder for the barium sulphate, various organic binders such as polyvinyl alcohol, polyvinyl acetate, polyvinyl isobutyl ether, ethyl cellulose, nitro cellulose, linseed oil alkyd and the like. These organic binders provide coatings which are highly reflective to ultraviolet light having wavelengths down to about 2,900 A.U., below which point the reflectance of these coatings begins to fall off and becomes rather low in comparison with the intrinsic reflectance of the virgin barium sulphate powder as the wavelength of light approaches 2,000 A.U. For most applications, this does not present too serious a problem since less than 4 percent of the solar energy is summed below the wavelength of 2,900 A.U.
For some unusual applications, however, the relatively low ultraviolet reflectance of the prior art barium sulphate coatings in the near vacuum ultraviolet is undesirable. Such an application involves the collection efficiency of a coating covering the internal surfaces of a chamber designed to detect Cerenkov radiation. To perform this function most efficiently, a coating, aside from reflecting electromagnetic energy dominantly in the diffuse mode, must possess the highest possible reflectance over that range of wavelengths of greatest interest for nucler charge detection. This range happens to include energy from 2,000 to 5,000 A.U. Hence, to meet the requirement of high reflectance in the neighborhood of 2,000 A.U., the prior art barium sulphate coatings which do not approach the intrinsic reflectance of the virgin barium sulphate powder in this region, are not satisfactory.